Bonding Si3 N4 ceramics

ABSTRACT

Compositions are disclosed for securing Si 3  N 4  parts together, these compositions being in the β&#39;-Y 2  Si 2  O 7  --Y 3  Al 5  O 12  tetrahedron, e.g. compositions in mole % comprising (1) 15% Si 3  N 4 , 79.1% Y 2  Si 2  O 7  and 6.9% Y 3  Al 5  O 12  or (2) 25% Si 2  AlON 3  and 75% Y 3  Al 5  O 12  Y. A method of securing Si 3  N 4  parts together comprises placing the composition between the parts to be bonded and heating to brazing temperature (about 1600° C.) in nitrogen.

TECHNICAL FIELD

This invention relates to compounds suitable for bonding or joiningceramic parts made of Si₃ N₄.

BACKGROUND ART

Parts made from Si₃ N₄ are expected to find extended use in hightemperature applications as, for example, in the hot parts of gasturbine engines or other load-carrying elements. To make such partsreadily usable there must be a suitable composition for joining theseparts to make composite structures that will maintain their integrity inuse and under the stresses to which the composite may be subjected. Suchcompositions must serve to join the said Si₃ N₄ pieces securely withoutdetrimentally affecting the pieces joined by the composition.

The β'-Y₂ Si₂ O₇ --Y₃ Al₅ O₁₂ compatibility tetrahedron establishes agroup of compositions that it was hoped could be usable as a brazingmaterial. Compatibility relationships in the Y--Si--Al--O--N system aredescribed in a report to NASA, Report No. NASA CR 159675, datedSeptember 1979 and also in a paper by Layden, No. 61-BN-79F, presentedat the Fall meeting in October 1979 to the American Ceramic Society.

DISCLOSURE OF INVENTION

One feature of the invention is a family of compositions capable ofsuccessfully joining Si₃ N₄ pieces together without any detrimentaleffect on the pieces. Another feature is a series of compositions thatmay be formed into a glass frit, rod, or sheet for use as a brazingmaterial between Si₃ N₄ parts. Another feature is a series ofcompositions that may be used as brazing material and that crystallizeupon suitable heat treatment to bond the parts together, thesecompositions having adequate wetting, flow, and mechanical compatibilitywith the Si₃ N₄.

According to the invention, compositions encompassed by the β'-Y₂ Si₂ O₇--Y₃ Al₅ O₁₂ tetrahedron are suitable as brazing or joining compositionsas they have the necessary characteristics such as wetting, flow andmechanical compatability with the Si₃ N₄ parts. More specificallycompositions in the tetrahedron Si_(3-x) Al_(x) O_(x) N_(4-x) (where0<x<2) --Y₂ Si₂ O₇ --Y₃ Al₅ O₁₂ have been found to be suitable brazingcompositions. Certain of these compositions, for example, composition 1:

    15 m/o Si.sub.3 N.sub.4 +79.1 m/o Y.sub.2 Si.sub.2 O.sub.7 +6.9 m/o Y.sub.3 Al.sub.5 O.sub.12

forms a viscous liquid that can be cooled from above the liquidustemperature to form glass that may be used as brazing material betweenSi₃ N₄ parts and other compositions, for example, composition 2:

    25 m/o Si.sub.2 AlON.sub.3 +75 m/o Y.sub.3 Al.sub.5 O.sub.12

which form low viscosity liquids at brazing temperature which tend tocrystallize on cooling. The use of the other rare earths alone or incombination with yttrium in the above composites will also producesuitable brazing material for Si₃ N₄.

BEST MODE FOR CARRYING OUT THE INVENTION

For the purpose of securing Si₃ N₄ parts together particularly in makingcomposite parts the bonding or brazing material must be compatible withthe material of the parts to be joined. The solid phase compatibility inportions of the Y--Si--Al--O--N system relevant to Si₃ N₄ and Si_(3-x)Al_(x) O_(x) N_(4-x) (where x is between 0 and 2) are readily determinedfrom the known models of the Y--Si--Al--O--N system which show the solidphase compatibility tetrahedra. As above stated, such models are shownand described in the NASA report, CR 159675, of September 1979 and aLayden paper, 61-BN-79F, delivered at the Fall meeting of the AmericanCeramic Society in October 1979.

A single tetrahedron encompasses compatibility with Si₃ N₄ over theentire range of Si_(3-x) Al_(x) O_(x) N_(4-x) (where 0<x<2)compositions. This is a tetrahedron β'-Y₂ Si₂ O₇ --Y₃ Al₅ O₁₂ and thisencompasses compositions exhibiting a wide range of solidus temperaturesand liquid viscosities. The compositions are found to be compatible withSi₃ N₄ in mechanical and structural respects, and the liquid phaseexhibits the desired wetting and flow characteristics.

For example, two compositions from this β tetrahedron representingextremes of a broad spectrum of liquid properties available in thistetrahedron at a temperature of 1600° C. are:

    (1) 15 m/o Si.sub.3 N.sub.4 +79.1 m/o Y.sub.2 Si.sub.2 O.sub.7 +6.9 m/o Y.sub.3 Al.sub.5 O.sub.12

    (2) 25 m/o Si.sub.2 AlON.sub.3 +75 m/o Y.sub.3 Al.sub.5 O.sub.12

Composition 1 represents a highly viscous liquid that cools to a glassand composition 2 is a fluid liquid that is expected to crystallize oncooling. These compositions are given in weight percent of actualconstituents as follows:

                  TABLE I                                                         ______________________________________                                        Composition                                                                              Si.sub.3 N.sub.4                                                                       AlN     Al.sub.2 O.sub.3                                                                     Y.sub.2 O.sub.3                                                                     SiO.sub.2                            ______________________________________                                        (1)        6.59     0        4.50  60.12 28.80                                (2)        4.88     0.71    41.55  52.87  0                                   ______________________________________                                    

In using these compositions for bonding parts made of Si₃ N₄, materialswere prepared in one gram batches and blended with a mortar and pestle,under methanol, to a smooth, creamy consistency. The Si₃ N₄ parts werepolished and dried with methanol and a thin layer of the methanol slurrywas placed between the parts to be joined. The assemblies were placed onboron nitride plaques and fired at 1600° C. in nitrogen at oneatmosphere for 10 minutes and then cooled. Upon visual and macrographicexamination, both compositions wet the Si₃ N₄ parts and producedwell-formed gussets at the braze joint.

Upon metallographic examination of the polished joints, composition 1showed some attack and penetration of the Si₃ N₄ surface, with goodbonding of the glass bonding composition to the surfaces. Composition 2showed more extensive reaction with the Si₃ N₄ than did composition 1and exhibited dendritic crystallization. Excellent mechanicalcompatibility of the braze joint was apparent in both instances. Whenbraze joints were impacted in such a fashion as to shear the brazejoint, in some instances the Si₃ N₄ parts failed while the braze jointremained intact, giving evidence of the superior strength of the joint.

Accordingly, it is believed that those compositions encompassed by theβ'-Y₂ Si₂ O₇ --Y₃ Al₅ O₁₂ tetrahedron are suitable as brazing or joiningcompositions as they would have the necessary characteristics such aswetting, flow and mechanical compatibility with the Si₃ N₄ parts. It isalso expected that the yttrium may be replaced in whole or in part byany of the other rare earth elements having similar characteristics toyttrium.

The tetrahedron would be the same as above with R (rare earth)substituted for Y. Thus the tetrahedron would be Si_(3-x) Al_(x) O_(x)N_(4-x) (where 0<x<2) --R₂ Si₂ O₇ --R₃ Al₅ O₁₂.

Although the invention has been shown and described with respect to apreferred embodiment thereof, it should be understood by those skilledin the art that other various changes and omissions in the form anddetail thereof may be made therein without departing from the spirit andthe scope of the invention.

I claim:
 1. A composition particularly adapted to joining Si₃ N₄ partscomprising, in mole percent, 15% Si₃ N₄, 79.1% Y₂ Si₂ O₇ and 6.9% Y₃ Al₅O₁₂.
 2. A composition particularly adapted to joining Si₃ N₄ partscomprising, in mole percent, 25% Si₂ AlON₃ and 75% Y₃ Al₅ O₁₂.
 3. Amethod of bonding Si₃ N₄ parts together comprising placing between theparts to be bonded a composition comprising 15% Si₃ N₄, 79.1% Y₂ Si₂ O₇and 6.9% Y₃ Al₅ O₁₂ and heating to brazing temperature in nitrogen.
 4. Amethod of bonding Si₃ N₄ parts together comprising placing between theparts to be bonded a composition comprising 25% Si₂ AlON₃ and 75% Y₃ Al₅O₁₂ and heating to brazing temperature in nitrogen.